The invention relates to a pressure relief valve to protect hydraulic assemblies from damaging overload, in particular to protect the hydraulic brace system from pressure surges and other suddenly occurring overload, comprising a valve housing with a connection, wherein the input and output sides of the valve housing are separated from one another by means of a closing part, which can be displaced against the force of a valve spring and comprises a seal, and are connected to one another when an overload situation occurs to drain the hydraulic fluid.
Such pressure relief valves are known, for example, from DE 199 46 848 A1. Especially in underground hard coal mining, shield construction frames having one or more hydraulic props are used to secure the hollow space, wherein said props have to be protected from the above-mentioned overload with pressure relief valves. Said pressure relief valves are designed and built such that a valve piston that is arranged displaceably in a bore is pushed against the spring retainer, wherein it displaces the spring retainer against the force of the spring with sufficiently high overload and hereby crosses over a sealing ring until the radial bores it comprises establish a connection between the blind hole in the valve piston and the outlets so that the hydraulic fluid can drain and relieve the entire hydraulic assembly. When the overload has been decreased, the valve spring pushes the spring retainer and hence the valve piston back into its starting position, so that the system pressure prevails in the now closed blind hole without being able to influence other parts of the pressure relief valve. The illustrated embodiment in DE 199 46 848 A1 contains a double piston and between them a damping chamber, which is supplied with hydraulic fluid when the valve is opened as a consequence of the overload, and hereby additionally supports the movement of the spring retainer. When the pressure relief valve is being closed again, said damping chamber then ensures that the valve spring can return into the starting position only in a damped state. This is intended to prevent wobbling at the same time, which can occur especially during the opening and closing operations. The disadvantage, however, is that said pressure-balancing chamber in the pressure-balanced state of the pressure relief valve can become empty so that an even filling is not guaranteed upon actuation of the valve. In particular, this type of pressure relief valve can also lead to damage resulting from hydraulic self-destruction, however it is especially disadvantageous that several closing parts are used in order to achieve the above-described, even opening and closing operations of the pressure relief valve.